In treatment processes for various workpieces such as semiconductor wafers and liquid crystal glass, ionizers are used to neutralize (destaticize) positive or negative charge on an electrostatically charged workpiece. Some ionizers utilize corona discharge while others utilize soft X-rays. The ionizers utilizing corona discharge are roughly classified into a direct-current ionizer and an alternating-current ionizer. In general, the direct-current type ionizer has needle-like positive discharge electrodes and negative discharge electrodes. When positive and negative high-voltages are applied to the discharge electrodes, corona discharge is generated at discharge parts of the electrodes to generate positive and negative ions. The positive and negative ions are blown by air onto a workpiece to neutralize positive or negative charge on the workpiece.
In some ionizers of this type, the positive discharge electrodes and the negative discharge electrodes are arranged in proximity to each other so that corona discharge can be generated by applying relatively low high-voltage. In this case, the positive ion sources and the negative ion sources are provided in proximity to each other.
Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2004-253192) and Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2004-253193) disclose a fan-type ionizer that uses a fan to produce an air flow. In the ionizer, the fan is provided in an air blowing port which opens in a case, and positive and negative discharge electrodes are provided at intervals of approximately 90 degrees in the circumferential direction of the air blowing port. Positive and negative ions generated by the discharge electrodes are blown onto a workpiece by an air flow from the fan.
In the ionizer disclosed in the documents, however, the positive and negative discharge electrodes are spaced apart from each other, and thus application of higher high-voltage to the discharge electrodes is required in order to generate corona discharge.
This voltage issue can be resolved by disposing positive and negative discharge electrodes 20A, 20B in proximity to each other, as shown, for example, in FIG. 10. In the fan-type ionizer, a fan 21 rotates to produce an air flow, which travels while swirling around the rotational center O of the fan 21 as a spiral flow. Therefore, if the positive and negative discharge electrodes 20A, 20B are positioned in proximity to each other, in particular at equal distances from the rotational center O of the fan 21, flows 22A, 22B of the generated positive and negative ions overlap each other while the ions are being carried by the spiral air flow, as indicated by the arrows in FIG. 10 in relation to one pair of the discharge electrodes 20A, 20B, and thus the positive and negative ions may easily recombine to be neutralized. As a result, the amount of ions that reach a workpiece may be reduced, causing a problem that a removal efficiency of electricity falls.